As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential and have long cycle lifespan and a low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway.
As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some such lithium secondary batteries are commercially available.
In particular, lithium secondary batteries used in EVs require high energy density, high short-term power output, and use for 10 years or longer under harsh conditions and thus need to have much higher safety and longer lifespan than existing small lithium secondary batteries.
In a lithium ion secondary battery used in conventional small batteries, in general, a cathode is formed of a lithium cobalt oxide having a layered structure such as LiCoO2 and an anode is formed of a graphite-based material.
Lithium cobalt oxides are currently widely used due to excellent physical properties such as excellent cycle characteristics as compared to LiNiO2 and LiMn2O4. However, such lithium cobalt oxides are disadvantageous in that cobalt (Co) is eluted at high voltage or under high-temperature conditions.
To address these problems, technology for partially substituting Co with Al, Mg, B, or the like or technologies for surface treatment of a lithium cobalt oxide with a metal oxide such as Al2O3, Mg2O, TiO2, or the like are known.
However, when Co is partially substituted with the above-described metals or a surface of a lithium cobalt oxide is coated with a metal oxide, specific capacity may decrease due to addition of a coating material that does not directly participate in charge and discharge reaction, and a metal oxide with very low electrical conductivity mainly constitutes the coating material, which results in reduced conductivity.
In addition, the coating process reduces active reaction area and thus interfacial resistance may increase and high-rate charge and discharge characteristics may be deteriorated.
Therefore, there is an urgent need to develop technology for fundamentally addressing these problems and enhancing high voltage lifespan characteristics of a lithium cobalt oxide.